The above cited patent applications and patents, along with the prior art cited therein, is referred to for further background of this invention.
In Applicant's prior patents, there is disclosed method and apparatus for uniformly coating the interior of pipe by forming a pocket of powered plastic, and pushing the pocket through the pipe. The pocket is of a density and length to cause the powdered plastic material to melt to the interior wall surface of the pipe as the pocket is pushed from the inlet to the outlet of the pipe. The powder contacting the pipe wall is melted as the pocket passes through the pipe interior. The more dense the pocket and the slower the pocket moves through the pipe, the thicker the plastic coating which is fused to the interior of the pipe will be. Accordingly, Applicant has heretofore discovered the phenomena that a uniform coating of synthetic plastic resin can be applied to the interior wall surface of a heated pipe by controlling the density and contact time between the wall surface and the moving pocket of plastic, as the pocket makes a single trip through the pipe.
In carrying out the above method, it is advantageous to provide a dense pocket of powdered plastic at the inlet of the pipe. The pocket should be slightly in excess of the amount of plastic required to coat the entire peripheral wall surface of the pipe in a single pass therethrough. It would therefore be desirable to measure the precise amount of powder required to coat the pipe interior so that a minimum amount of residual powdered plastic emerges from the outlet end of the coated pipe. Moreover, it would be advantageous to be able to rapidly form a more dense pocket of precisely measured powdered plastic material, and to force the powdered material to move through the pipe interior in a minimum length of time so that the production rate of plastic coating the pipe can be held at some satisfactory elevated level of operation.
Method and apparatus for achieving the above is the subject of this invention.